The objective of this study is to discover what endows an object, appearing as an abrupt onset, with the capacity to capture attention. We examined the situation in which attention had already been prioritized to a target location when the abrupt onset, which always materialized in a distructor location, appeared. Neo and Chua (2006) showed that so long as the onset object was presented infrequently, attention that had been prioritized could still be captured by the onset. In this study, the following experimental logic was used: we preview the object that would later appear as an onset. The question was whether the preview would modulate the capacity of an abrupt onset in capturing attention. To discover just which aspects of the onset was critical, we previewed different aspects of the critical stimulus in separate experiments. To the extent that a feature is critical in attentional capture, previewing that feature should impair attentional capture. In Experiment 1, the critical stimulus (both its form [four spots] and mode of appearance [an abrupt onset]) was previewed. Capture was eliminated, which implied that the preview logic worked. In Experiment 2, we previewed only the form of the critical stimulus but not its mode of appearance. In Experiment 3, the critical stimulus's mode of appearance was previewed (essentially, the local luminance transients that accompany the onset). The critical stimulus in both experiments failed to capture attention, suggesting that both the form and the mode of appearance of the critical stimulus had to be previewed before its attention-capturing capacity would be undercut.